An umbilical comprises a group of one or more types of elongated functional elements such as electric cables, optical fibre cables, or hoses for fluid transportation of, for example, gas, water or chemical products such as methanol. The functional elements can be assembled together in a helical or S/Z manner and over-sheathed and/or over-armoured for mechanical strength and ballast.
It is desirable for a single umbilical to be able to contain as many functional elements as are required for a particular application, for example, as are required for a particular oil field where the umbilical is intended for use. Umbilicals are typically used for transmitting power, signals and fluids (for example for fluid injection, hydraulic power, gas release, etc.) to and from a subsea installation.
API Specification 17E/ISO 13628-5 “Specification for Subsea Umbilicals” provides standards for the design and manufacture of such umbilicals.
Subsea umbilicals are installed at increasing water depths, commonly deeper than 2000 m. Such umbilicals have to be able to withstand severe loading conditions during their installation and their service life.
The main load bearing components in charge of withstanding the axial loads due to the weight and to the movements of the umbilical are steels tubes (see U.S. Pat. No. 6,472,614, WO93/17176 and GB2316990), steel rods (see U.S. Pat. No. 6,472,614), composite rods (see WO2005/124213), or tensile armour layers (see FIG. 1 of U.S. Pat. No. 6,472,614).
The other elements, i.e. the electrical and optical cables, the thermoplastic hoses, the polymeric external sheath and the polymeric filler components do not contribute significantly to the tensile strength of the umbilical.
Electrical cables used in subsea umbilicals fall into two distinct categories respectively known as power cables and signal cables.
Power cables are used for transmitting high electrical power (typically a few MW) to powerful subsea equipments such as pumps. Power cables are generally rated at a medium voltage comprised between 6 kV and 35 kV. A typical power cable is illustrated in the accompanying FIG. 1. From inside outside, it comprises a central copper conductor 2a, semi-conductor and electrical insulation layers 2b, a metallic foil screen 2c and an external polymeric sheath 2d. The central conductor 2a has generally a stranded construction and a large section typically comprised between 50 mm2 and 400 mm2. Three phase power is provided by three such cables bundled together within the umbilical structure.
Signal cables are generally used for transmitting signals and low power (<1 kW) to electrical devices on the seabed. Signal cables are generally rated at a voltage smaller than 3000V, and typically smaller than 1000V. Signal cables generally consist of small section insulated conductors bundled together as pairs (2), quads (4) or, very rarely, any other number, said bundle being further over-sheathed. An example of quad signal cable is illustrated in the accompanying FIG. 2. Four small size stranded copper conductors 3a are individually over sheathed by polymeric insulation layers 3b and helically bundled together. A polymeric filler material 3c is added to fill the voids in the bundle and achieve a cylindrical shape. This arrangement is optionally surrounded by an electromagnetic shielding 3g made from a wrapped copper or aluminium foil. A polymeric external sheath 3d protects the cable against mechanical damage and water ingress.
A problem with known electric cables is the presence and migration of water and gas along the electrical cable conductor. Water and gas can permeate through polymer sheaths and insulation layers and then migrate along the cable conductor to subsea terminations and potentially lead to premature failure. Gas can also migrate to the topside junction boxes potentially creating hazards if not vented away.
In particular, hydrogen formation can occur where there are components comprising zinc within the umbilical, for example zinc coated steel armours. If hydrogen forms within the umbilical, then the hydrogen gas will try to find a way to exit the umbilical. Sometimes it finds a way through the external sheath of the umbilical. However, it has also been observed the hydrogen could permeate through the electrical cables' outer sheath and insulation layers to reach the electrical conductors and then propagate along the conductors towards the end of the umbilical. At the end of the umbilical, the hydrogen typically may become backed-up and may begin to build pressure (if the termination is not vented). This may lead to an explosion and/or a loss of electrical insulation (short circuit).
WO2008/032019 describes adding a tubular metallic layer between the insulation layer and the external sheath of the electrical cables, in order to prevent water and gas permeation. However, delicacy is needed to implement such a tubular metallic screen which needs both to be impermeable enough to hydrogen (small size molecules) and flexible enough to withstand bending loads.
WO2008/032019 also describes filling the voids/interstices between the strands of the conductor with a hydrogen absorbent material. However, this has a service life issue: when this material has fully reacted with hydrogen, the protection does not work anymore, although it can be used in combination with the tubular metallic screen to reduce as much as possible the amount of hydrogen to neutralize.
WO2009/064559 describes monitoring the hydrogen migration along the cables and relieving pressure when it reaches a predetermined amount (page 3, end of §[0005]). However, this is not fully reliable, and is difficult to implement on subsea terminations.
U.S. Pat. No. 7,285,726 describes a subsea power cable comprising a stranded copper conductor. The voids between the strands are filled with a hydrophobic water-blocking compound in order to prevent longitudinal water penetration and facilitate repair (col. 3, lines 20-23). However, this can lead to manufacturing problems as the jelly compound may gas out during the insulation extrusion process.
One object of the present invention is to minimise or overcome these problems.